Flattened mountain tops
One extra example out of many:
A flattened mountain today.
A flattened mountain top in ancient times on Youtube.
but guessing about the flattened mountain tops in Nazca.
Fact is: they are there. And probably flattened hilltops
were in many places on earth (lets
keep in mind the Acropolis or Monte Alban). But many 'alien flattened tops' are
surely eroded now or full of modern buildings, but
we would not be surprised if 'the aliens' used them once to place telescopes or rockets on it ... just like
we do now.